This invention relates to cable transducers and more particularly to such transducers that are buried in the ground for detection of ground disturbances. One application of the invention is in intrusion detection systems.
The coaxial cable transducer described in U.S. Pat. No. 3,763,482 has been very successful in fence applications where the sensor cable is clamped directly to the fence body. Cable sensitivity to vibrations induced in the fence structure by intruders attempting to climb it is high partly because of the efficient transfer of mechanical energy from the fence to the cable. However, when the cable is buried in the ground as a sensor for detecting surface disturbances such as an intruder's footsteps, the ground motion is relatively uniform along the cable surface so that the cable tends to move with it without producing significant displacement between the conductors and thus, in effect, reducing the sensitivity of the cable to the seismic energy. An additional factor is the lack of penetration of the surface pressure beneath a footstep due to the finite stiffness of the intervening material. The degree of this effect depends upon the cohesiveness of the ground and will vary, for example, between frozen and unfrozen ground, the cable being less sensitive under the latter condition.
A principal advantage of the cable transducer described in the foregoing patent is that it may be produced from conventional coaxial hookup wire at relatively low cost per unit length. Such hookup cable, however, is not designed to withstand continuous exposure to moisture and other corrosive elements present in the ground. Since moisture penetration of the outer jacket of the cable impairs or destroys its sensitivity, special moisture-proofing treatment of the cable is required to adapt it for buried installations at higher costs and with the possible additional disadvantage of reduction of sensitivity.
Another inexpensive electrostatically charged cable capable of detecting vibrations, pressure waves and the like mechanical disturbances is the twisted pair cable described in copending application of M.D. Laymon and G.K. Miller, Ser. No. 718,845, assigned to the assignee of this invention. This cable is likewise susceptible to the adverse effects of moisture penetration when buried in the ground.